Strain gauge pump control switch

ABSTRACT

A control switch incorporates a solid state transducer, a strain gauge. The transducer responds to a local environmental condition, such as fluid level, or pressure and exhibits a parameter change which can be detected as an electrical output. Control circuits coupled to the transducer can sense the parameter change and switch a source of electrical energy to a load in response thereto.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent application Ser. No. 13/956,000 filed Jul. 31, 2013 and entitled “Strain Gauge Pump Switch” which is a divisional of U.S. patent application Ser. No. 12/123,850 filed May 20, 2008 and entitled “Strain Gauge Pump Switch” which claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/939,453 filed May 22, 2007 and entitled “Strain Gauge Pump Switch”. The disclosures of these applications are fully incorporated herein by reference.

FIELD

The invention pertains to solid state pump control switches. More particularly, the invention pertains to such switches which incorporate a strain gauge as a transducer to convert an environment condition, such as a level of a fluid, to an electrical signal.

BACKGROUND

Various types of switches have been developed for use in turning pumps on and off in response to an external ambient condition, such as water level. Such switches tend to be used in relative harsh environments such as in tanks of water, or, sump pits which are used to collect foundation water. Other environments include industrial fluids which might be caustic or acidic, as well as high or low temperatures.

While known switches can be useful and function properly over a period of time, they are always subject to failure. Switch failures in turn translate into non-running pumps which can result in flooded commercial, industrial and residential locations. Alternately, non-running pumps can result in water supply deficiencies, or failures to supply commercial or industrial fluids for various applications.

One switch configuration has been disclosed in U.S. Pat. No. 7,307,538, issued Dec. 11, 2007, and entitled “Pump Connector System”. The '538 patent is assigned to the assignee hereof and is incorporated herein by reference.

There is an on-going need for control switches usable in such environments which exhibit greater reliability and longer lifetimes than do known switches. Preferably, such improved switches would be price competitive with known switches and readily substitutable therefore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a switch assemblage in accordance with the invention;

FIG. 2 is a side sectional view of an exemplary switch housing as in FIG. 1;

FIG. 3 is an end view of an embodiment of a transducer in accordance with the invention; and

FIG. 4 is a block diagram of control circuits in accordance with an embodiment of the invention. FIG. 4A is a perspective view of an electrical cable and AC connector in accordance with the invention.

DETAILED DESCRIPTION

While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated.

Embodiments of the invention incorporate a strain gauge as a transducer to sense the presence of a fluid either through displacement, buoyancy of a structure or by fluid pressure deforming a strain gauge platform. Such embodiments can be implemented as solid state structures which can be digitally calibrated for various settings or pressure.

In one aspect of the invention, such transducers can be coupled to electronic control and switching circuitry which can switch electrical energy to activate a load, such as an electric motor for a pump. Advantageously, motor starting inrush currents are diverted away from the transducers in such embodiments. Further, such transducers can accurately respond to changing conditions, such as level or pressure, resist vibration and can withstand harsh operating environments.

The control and switching circuitry can include relatively high power semiconductor switches which are controlled by one or more programmable processors which in turn are coupled to one or more solid state transducers, preferable strain gauges. The processor(s) can digitally calibrate one or more strain gauges.

In another aspect of the invention, a solid state switch, such as a triac, can be coupled in parallel with a relay to a pump motor connector. The switch and relay can be independently controlled by control circuits in the unit. In response to signals from the strain gauge, the control circuits can bias the switch to a low impedance state to couple electrical energy to the pump connector. In this state, the switch can couple the motor start up, inrush, current without arcing or the like to the pump connector to start the motor. Once the inrush currents have subsided, for example after a time interval such as two or three seconds, the control circuits can activate the relay which changes state and provides a closed contact pair to carry the motor current as an alternate to the solid state switch. The relay contacts shunt the motor current away from the switch enabling it to cool off as needed.

When the strain gauge indicates that the lower water level has been reached, the control circuits de-energize, turn off, the relay which open circuits the motor current circuit through those contacts. Subsequently, after another time interval, such as two or three seconds, the solid state switch is biased off, or placed in a high impedance state by the control circuits. When the switch turns off it, and not the relay contacts, absorbs any turn off current or voltage transients which might otherwise cause arcing at the relay contacts. Those contacts are thus protected from electro-ablation, contact burning.

With respect to FIGS. 1-4, a pump control system 10 includes a water tight housing 12 with an open end 12 a closed by a retaining ring 14 which carries an axially movable pressure sensing plate 16. Sensing plate 16 carries a connector prong 30, best seen in FIG. 3. Housing 12 is placed in the sump along with a pump to be switched on and off.

An electrical cable 20 couples housing 12 to a double sided AC connector 22. Connector 22 carries a pump AC receptacle 22 a at one end and AC outlet prongs 22 b at the other end. In operation, a pump AC connector is plugged into receptacle 22 a. Prongs 22 b are plugged into a local utility supplied AC outlet.

Ring 14 has an annular shape with molded radial members 14-1, -2, -3, and -4. Radial member 14-4 carries an elongated, deflectable, metal plate 32 which supports a strain gauge 34. A centered perforation 36 in a free end of plate 32 receives the connector prong 30 with a friction fit and supports pressure plate 16 for axial motion in response to applied fluid pressure.

A printed circuit board 40 carries sensing and control circuits 42. Surge suppressing circuits 44 are coupled to a DC supply 46. A digital circuit regulator 48 and analog circuit regulator 50 feed digital circuits 56 and differential amplifier 52 respectively.

The differential amplifier 52 is coupled to strain gauge 34 via connectors 34 a. Movement of the plate 16 in a first direction in response to increasing fluid pressure generates a signal of a first polarity at amplifier 52. Movement of plate 16 in the opposite direction, in response to decreasing fluid pressure generates a signal of the oppose polarity at amplifier 52.

Digital control circuits 56 include a programmable processor or computer 58 a, and associated storage, random access memory, EEPROM and Flash memory indicated generally at 58 b. Software, or, control programs stored in EEPROM or Flash memory can be executed by processor 58 a in carrying out the above described switching process.

Circuits 56 can be accessed via a programming interface 58 e. A factory calibration port 58 f is also provided.

Digital output circuits 58 c are respectively coupled to Triac driver 62 a and Triac 62 b, and relay driver 64 a and relay 64 b. As described above, electrical energy from connector 22 is switched by Triac 62 b and relay 64 b to provide a switched AC output 20 c which can be coupled to pump motor 24 via pump receptacle end 22 a.

A vent tube 20 d extends from within housing 12, via cable 20 and terminates at connector 22. Tube 20 d maintains pressure in the housing 12 at local atmospheric pressure.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. 

The invention claimed is:
 1. A control system for an electric motor comprising: a watertight housing; a strain gauge carried in the housing; control circuits, carried at least in part in the housing, coupled to the strain gauge and responsive to an output of the strain gauge; a relay and an electronic switch coupled to and independently controlled by the control circuits; an electrical connector for a motor, the electrical connector for the motor having prongs and receptacles wherein the electronic switch and the relay are directly electrically connected in parallel between at least one of the prongs and at least one of the receptacles of the electrical connector for the motor and, the control circuits, responsive to the output of the strain gauge, sequentially: (i) place the electronic switch into a conducting state to couple an electric motor start up, inrush current, without arcing, from the at least one prong, through the electronic switch, to the at least one receptacle to start the motor; and (ii) once the inrush current has subsided, the control circuits change the state of the relay to provide a closed contact pair that carries the motor current from the at least one prong, through the relay, to the at least one receptacle as an alternate, parallel path to operate the motor.
 2. A control system as in claim 1 which includes pump turn-off software responsive to at least one electrical signal from the strain gauge.
 3. A control system as in claim 2 where the pump turn-off software includes; causing the relay to return to the non-conducting state; establishing a selected time interval; responsive to a termination of the selected time interval, causing the electronic switch to enter a high impedance state.
 4. A control system as in claim 1 wherein the electronic switch is placed into a non-conducting state subsequent to the relay open circuiting the closed contact pair.
 5. A control system as in claim 4 which includes a vent tube to maintain pressure in the housing at local atmospheric pressure.
 6. A control system as in claim 1 which includes a bridge circuit, at least one branch of the bridge circuit is coupled to a differential amplifier.
 7. A control system as in claim 1 where the control circuits include a programmable processor and associated control software stored on a computer readable medium.
 8. A control system as in claim 7 which includes first software that monitors at least one electrical signal from the strain gauge.
 9. A control system as in claim 8 which includes second software, responsive to at least one electrical signal from the strain gauge that carries out a pump turn-on sequence.
 10. A method of controlling an electric motor comprising: providing a vented housing; locating a pressure responsive at least partly mechanical switch in the housing; coupling a relay and an electronic switch electrically in parallel; responsive to an output from the pressure responsive at least partly mechanical switch, placing the electronic switch into a conducting state to couple an electric motor start up, inrush current, substantially without arcing, to at least one receptacle to start a motor; and once the inrush current has subsided, changing the state of the relay and providing a closed contact pair that carries a motor current through the relay, thereby providing an alternate, parallel path for the motor current.
 11. A method as in claim 10 which includes providing a vent tube with one end in the housing.
 12. A method as in claim 10 which includes, providing in the pressure responsive switch an axially movable pressure responsive member.
 13. A method as in claim 12 which includes responding to movement of the pressure responsive member, in one direction, thereby placing the electronic switch into the conducting state.
 14. A method as in claim 13 which includes responding to movement of the pressure responsive member, in a different direction, thereby open circuiting the electronic switch.
 15. A method as in claim 14 which includes open circuiting the relay prior to open circuiting the electronic switch.
 16. A method as in claim 15 which includes providing a vent tube with one end in the housing.
 17. A method as in claim 15 which includes providing a predetermined time interval which enables the inrush current to subside prior to changing the state of the relay.
 18. A method as in claim 17 providing a second predetermined time interval and changing the state of the electronic switch in response thereto. 